(a) Field
The described technology relates generally to a sounding method. More particularly, the described technology relates generally to a sounding method in a wireless local area network (WLAN).
(b) Description of the Related Art
A WLAN transmits or receives data using an unlicensed band of 2.4 GHz or 5 GHz and is being standardized by the IEEE (Institute of Electrical and Electronics Engineers) Part 11 under the name of “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.”
After an original standard was published in 1999, new version standards are continuously published by amendments. The IEEE standard 802.11a (IEEE Std 802.11a-1999) supporting 2.4 GHz band was published in 1999, the IEEE standard 802.11b (IEEE Std 802.11b-1999) supporting 5 GHz band was published in 1999, and the IEEE standard 802.11g (IEEE Std 802.11g-2003) supporting 5 GHz band was published in 2003. These standards are called legacy. Subsequently, the IEEE standard 802.11n (IEEE Std 802.11n-2009) for enhancements for higher throughput (HT) was published in 2009, and the IEEE standard 802.11 ac (IEEE 802.11 ac-2013) for enhancements for very high throughput (VHT) was published in 2013. Recently, a high efficiency (HE) WLAN for enhancing the system throughput in high density scenarios is being developed by the IEEE 802.11ax task group.
The HE WLAN or a subsequent WLAN may use a multi-user transmission. For example, a device may simultaneously transmit data to a plurality of devices or the plurality of devices may simultaneously transmit data by using a scheme such as orthogonal frequency division multiple access (OFDMA).
For the multi-user transmission, a given bandwidth may be divided into a plurality of subchannels and the plurality of subchannels may be allocated to multi users. In this case, a scheme such as beamforming may be applied to the plurality of subchannels to improve the transmission gain. For this, a sounding procedure is required for the plurality of subchannels.